Allow CE do nothing if route doesn't exist

doc/development/ee_features.md

@@ -338,13 +338,14 @@ When we add `draw :admin` in `config/routes.rb`, the application will try to
 load the file located in `config/routes/admin.rb`, and also try to load the
 file located in `ee/config/routes/admin.rb`.
 
-It should at least load one file, at most two files. If it cannot find any
-files, an error will be raised.
+In EE, it should at least load one file, at most two files. If it cannot find
+any files, an error will be raised. In CE, since we don't know if there will
+be an EE route, it will not raise any errors even if it cannot find anything.
 
 This means if we want to extend a particular CE route file, just add the same
 file located in `ee/config/routes`. If we want to add an EE only route, we
-could still use `draw :ee_only` and add `ee/config/routes/ee_only.rb` without
-adding `config/routes/ee_only.rb`.
+could still put `draw :ee_only` in both CE and EE, and add
+`ee/config/routes/ee_only.rb` in EE, similar to `render_if_exists`.
 
 ### Code in `app/controllers/`
 

ee/config/routes/oauth.rb

+# frozen_string_literal: true
+
 namespace :oauth do
   scope path: 'geo', controller: :geo_auth, as: :geo do
     get 'auth'

ee/lib/ee/gitlab/patch/draw_route.rb

+# frozen_string_literal: true
+
+module EE
+  module Gitlab
+    module Patch
+      module DrawRoute
+        extend ::Gitlab::Utils::Override
+
+        override :draw_ee
+        def draw_ee(routes_name)
+          draw_route(route_path("ee/config/routes/#{routes_name}.rb"))
+        end
+      end
+    end
+  end
+end

ee/spec/lib/gitlab/patch/draw_route_spec.rb

@@ -39,4 +39,9 @@ describe Gitlab::Patch::DrawRoute do
       .with(File.read(subject.route_path('ee/config/routes/admin.rb')))
       .once
   end
+
+  it 'raises an error when nothing is drawn' do
+    expect { subject.draw(:non_existing) }
+      .to raise_error(described_class::RoutesNotFound)
+  end
 end

lib/gitlab/patch/draw_route.rb

@@ -5,6 +5,8 @@
 module Gitlab
   module Patch
     module DrawRoute
+      prepend EE::Gitlab::Patch::DrawRoute
+
       RoutesNotFound = Class.new(StandardError)
 
       def draw(routes_name)
@@ -16,8 +18,8 @@ module Gitlab
         draw_route(route_path("config/routes/#{routes_name}.rb"))
       end
 
-      def draw_ee(routes_name)
-        draw_route(route_path("ee/config/routes/#{routes_name}.rb"))
+      def draw_ee(_)
+        true
       end
 
       def route_path(routes_name)

spec/fast_spec_helper.rb

@@ -8,3 +8,4 @@ require_relative 'support/rspec'
 require 'active_support/all'
 
 ActiveSupport::Dependencies.autoload_paths << 'lib'
+ActiveSupport::Dependencies.autoload_paths << 'ee/lib'